1. Field of the Invention
The present invention relates to electrical and electronic circuits and systems. More specifically, the present invention relates to systems and methods for receiving GPS (Global Positioning Systems) and other radio frequency signals.
2. Description of Related Art
In GPS signal reception, the signal at the antenna is at very low levels. In fact for most normal GPS antennas, the received signal, being a spread spectrum signal, is significantly below the thermal noise level in a receiver front end.
Further, to implement a low cost controlled reception pattern antenna (CRPA) with conventional teachings, one of the major cost drivers is the cost of the antenna elements themselves. Often there is limited space for mounting the elements as well. Theory conventionally indicates that with N antenna elements, one can theoretically form N−1 nulls in independent directions of arrival to minimize interference. For example, a 3 element antenna (N=3) can form two (N−1) independent nulls to minimize interference. This is accomplished by changing the weights and phases (or alternatively changing the weights of the in-phase and quadrature components) of each antenna element output and then summing the resulting weighted element outputs together. Thus, the ability to null jammers and other sources of interference with a lower number of antenna elements is usually an advantage.
Traditionally, some GPS antenna elements receive a right hand circularly polarized signal by internally receiving two linear polarizations such as the vertical and horizontal polarization components in separate feed lines and then combining these two polarization components to output a circularly polarized signal. GPS patch elements commonly use this approach. However, this approach is costly and requires too much space for certain applications such as small (4–5 inch diameter) GPS guided projectiles. Hence, there is a need in the art for a system or method for effecting spatial nulling of GPS signals for small projectiles.
In addition, because the signal level is low (total power at the antenna is typically 120 dB below one milliwatt), the potential for interference, either inadvertent or deliberate, is great. One way to counter this interference is with an adaptive antenna designed to minimize the power from an interfering source. The adaptive antenna will tend to minimize the antenna gain in the direction of the arriving interference.
However, for an adaptive antenna, the controlling algorithm must be constrained in some way to counter the tendency for the antenna to minimize the interference by shutting off all antenna elements. This is counterproductive since the goal is to successfully receive the GPS satellite signals. A common constraint is to require one of the antenna elements be weighted to near its maximum gain state at all times at a fixed value. This thereby prevents the antenna algorithm from shutting off the desired GPS signal.
One problem with this approach is that it takes away some of the degrees of freedom for an antenna for certain interference directions. Another problem with fixing the gain of one element nearly fully on is that for some interference geometries, the desired weights are such that the reference “fixed” element weight should have less gain than one or more of the other elements. When the reference element is fixed to nearly fully on, hardware limitations prevent providing gains for the other elements that are much greater than the reference element. For these geometries, this results in loss of some of the degrees of freedom. This means that a 3 element antenna cannot do a good job of nulling two interferers for certain geometries.
Further, traditional spatial nulling systems had difficulty nulling jammers from certain combinations of movement and angles relative to the receiving elements. Accordingly, there is an additional need in the art for a system and method for improving the performance of GPS receivers with respect to jammers.